1. Field of the Invention
This invention relates to a device for the automatic control of the vibration frequency and/or amplitudes of a variable-moment vibrator.
2. Description of the Prior Art
The very principle of vibratory pile driving is known to consist in making the ground vibrate by means of the object to be driven in. In particular, in soils of low consistency, the ground becomes fluidized thus enabling the profile to be driven into the ground by way of its own weight combined with that of the vibrator working above it.
Usually, vibrators used for pile driving use at least two trains of eccentric feeders mounted rotatably about shafts integral with two respective pinions meshing with one another so as to rotate in opposite directions to one another, under the effect of a motorization which can comprise one or more motors.
The variation of the vibratory moment generated by the two trains of feeders is obtained by means of a phase shifter capable of generating a variable angular phase shift between the rotary motions of the two feeder trains.
It so happens that vibrations propagate from the place of pile driving and the intensity of these vibrations can be harmful to neighboring buildings, especially to masonry work with little bonding or to premises housing equipment that is sensitive to vibrations, such as measuring instruments and/or computers.
The main object of this invention is to remedy the preceding disadvantages, particularly to permanently control the amplitude of vibrations in order to remain below a threshold of vibration-induced harmfulness, this threshold being variable as a function of the type of ground, of the objects to be driven in, and of the installations to be protected. It notes that according physical laws, the harmfulness of the vibrator for the vibrations emissions, depends on the emitted vibratory energy and therefore of the amplitude product by the frequency.
Accordingly, there is provided an automatic control device using at least one accelerometric detecter positioned so as to be directly or indirectly subjected to the vibrations generated by the vibrator, a means for deducing a value representing the harmfulness of the vibrations on the basis of the accelerometric data provided by the accelerometric detector, a means for entering a set value, and an electronic processing means associated with an actuating means acting on the phase-shift control of motorization speed variation control and therefore on the vibrations frequency and/or of the phase shifter in order to maintain the value representing the harmfulness of the vibrations at a level equal to or lower than the set value.
By way of these arrangements, subsequent to a few on-site trials conducted beforehand, the operator can determine a set value which, once it has been entered, will ensure the generation of a vibration of sufficient amplitude for driving into the ground but which will remain below the threshold of harmfulness pertaining to sensitive installations in the vicinity. In the same way, these tests will permit to determinate and to memorize a harmfulness/amplitude law, said law being used by the device for the automatic control to set the vibrations amplitude and/or frequency in order to maintain the harmfulness of the vibrations at a level equal to or lower than the set value.
Advantageously, the accelerometric detector is positioned on the vibrating shell of the vibrator. In this case, this detector can be linked to the electronic processing means via a wire connection.
However, the invention is not limited to this solution. In fact, the device embodying the invention can use an accelerometric station fitted with an electronic processing means and to be installed on the structure to be protected. In this case, the transmission between the accelerometric station and the electronic processing circuits can be carried out by any remote transmission means such as a wire connection, radio, etc.